The present invention relates to a wet spinning process for obtaining polyvinyl alcohol (hereinafter referred to as PVA) fiber containing boric acid or a borate, and more especially to a very stable spinning process for producing a PVA fiber of high tenacity and high modulus.
The process of spinning a PVA solution containing boric acid or a borate into an alkaline coagulation bath to produce PVA fiber has long been known. For example, the specifications of Japanese Patent Publications No. 8918/1956 and No. 2061/1959 disclose the process of spinning a PVA spinning solution containing boric acid or a borate into an alkaline coagulation bath containing different salts.
It is supposed that in these processes the PVA spinning solution may form a fiber by the gelation reaction with the alkaline component, by the dehydrating coagulation reaction with the salts of the coagulation bath and further by the crosslinking reaction with boric acid. Since these reactions proceed simultaneously, the mechanism of the spinning reaction is so delicate that it is affected by slight changes in the spinning conditions and becomes very unstable. Accordingly, the quality of the thus obtained products fluctuates notably and the tendency for the products to degenerate with the lapse of time is extremely apparent and is one of the largest problems associated with this process. Spinning methods for PVA fibers of the above-mentioned type have long been known, and a sufficient number of basic studies have been reported to enable production of fibers of relatively high quality, but this technique has not yet been applied practically to industrial production, evidently because of the above-mentioned unsteadiness of spinning and the unstable quality.
The present inventors, after researching the causes of such unsteadiness of spinning and instability also taking into consideration the change in the quality of the fiber with the lapse of time, have found that these deficiencies are a function of the metal deposit on the backside of the spinning nozzle (on the side of the spinning solution) and of the deposit of scale around the nozzle orifice of the front side of the spinning nozzle (on the side of the coagulation bath).
Since the spinning nozzle for a wet process is generally made of an alloy of gold with platinum, the noble metal when coming in contact with some non-noble metal is apt to create a galvanic cell and itself become a cathode while making the non-noble metal an anode.
The spinning solution contains a fair amount of metal ions derived from the water solvent or from erosion of the apparatus, which metal ions appear to have been deposited on the nozzle acting as a cathode. According to the result of our analysis, the deposited metal consists principally of copper and subsidiary components such as iron.
According to the results of our analysis, it has been found that the scale deposited around the nozzle orifice contains iron and calcium compounds. This fact suggests that the metal ions contained in the spinning solution would become insoluble compounds to deposit as the scale around the nozzle orifice.